Perception, motion, and cognition involve the formation of cooperative neuronal assemblies distributed over the cerebral cortex. It remains to explore what characterizes the assemblies, their location, and the structural substrate of assembly formation. In this EEG/fMRI study, we describe the response of the visual areas of the two hemispheres in subjects who viewed bilateral iso-oriented (IG) or orthogonally-oriented (OG) moving gratings projected in the two hemifields. The IG stimulus synchronized activity across the hemispheres, as shown by an increased EEG coherence. The increase was restricted to the occipital electrodes and to the beta band. Compared with OG, IG increased the BOLD signal in a restricted territory corresponding to area VP/V4. Within this territory, a linear relation was found between the increased interhemispheric EEG coherence and BOLD. Thus, the increased BOLD localized a trans-hemispheric, synchronous neuronal assembly probably achieved by a callosal cortico-cortical connection. This assembly might reflect an early stage of perceptual grouping since the IG stimulus conforms to Gestalt psychology principles of collinearity and common fate.